Kinetics of the base-catalyzed permanganate oxidation of benzaldehyde

The kinetics of the base-catalyzed permanganate oxidation of benzaldehyde have been reexamined. The rate is proportional to the first power of the aldehyde and permanganate concentrations, and there are terms that are zero order, first order, and second order in hydroxide ion. The reaction has an isotope effect, and the effect of substituents gives rho = +1.58. The possible mechanisms for the reaction are discussed in the context of ab initio calculations at the B3P86/6-311+G and MP2/6-31G theoretical levels, and both one- and two-electron processes are possible. Benzaldehyde hydrate dianion is calculated to have a remarkably small C-H bond dissociation energy.     

FTIR spectrum of benzene in a supersonic expansion

The 700 and 3050 cm^?1 region of benzene seeded in argon is studied using Fourier-transform infrared absorption spectroscopy. Analysis of the sequence band structure in the 700 cm^?1 region gave new anharmonic constants and values of T_rot (79 ± 15 K) and T_vib (/2~160 K) which indicate a disequilibrium between the rotational and vibrational degrees of freedom. A band contour analysis of the 3050 cm^?1 Fermi triad is used to obtain new values of the Coriolis coupling constants and band origins.     

Disilynes. III [1] A Relatively Stable Disilyne RSi≡SiR (R = SiMe(SitBu3)2)

The disilyne R**Si≡SiR** (R** = SiMe(SitBu₃)₂), prepared as the first isolable and realtively stable silicon compound with a SiSi triple bond two years ago by dehalogenation of trans‐R**ClSi=SiClR** with LiC₁₀H₈ in thf at ‐78 °C (calc.: Si≡Si distance 2.072Å, Si‐Si≡Si bond angle 148°), forms with CH₂=CH₂ a [2+2] and with CH₂=CH‐CH=CH₂ a [2+4] cycloadduct. The ethene adduct takes up oxygen very easily with change of the Si=Si group into a SiOSiO ring with formation of R**Si(μ‐O)(μ‐O)(μ‐C₂H₄)SiR**. By heating the disilyne in heptane to ca. 50 °C in the presence of traces of thf it transforms into a monoxide of the ethene adduct with formation of R**Si(μ‐O)(μ‐C₂H₄)SiR**. In thf, the disilyne rearranges at r.t. and below by migration of a SitBu₃ group with formation of a silyl substituted cyclotrisilene. X‐ray structure determinations of the ethene adduct and its mono‐ and dioxide are presented.     

Significant kinetic solvent isotope effects in folding of the catalytic RNA from the hepatitis delta virus

The exchange of deuterium for hydrogen in water often produces solvent kinetic isotope effects (KSIEs) on the rate constants associated with enzyme reactions, including those catalyzed by RNA. Recently, KSIEs have been used to show that proton transfer occurs in the rate-limiting step of cleavage by the hepatitis delta virus (HDV) ribozyme and other catalytic RNAs. To test the underlying assumption that KSIEs are related to the chemistry step of ribozyme-mediated cleavage reactions, we developed fluorescence resonance energy transfer assays to measure KSIEs on the rate constants of conformational changes associated with substrate binding and dissociation by a trans-acting HDV ribozyme. We observe comparable KSIEs ( approximately 2-2.5-fold) of rate constants of conformational change and cleavage, while proton inventory experiments are consistent with a shift in the ensemble of transition states upon increase of D2O in the solvent. Taken together, these results challenge the common assumption that pL profiles of RNA-catalyzed reactions yielding a pKa and KSIE necessarily provide evidence for an ionization (chemistry) step to be rate-limiting. They also suggest that an unusual proton inventory may provide a signature for a conformational change contributing to the rate-limiting step.     

The effect of hydration/dehydration on the AC electrical conductivity of H4PVMo11O40 and some of its cesium salts used as catalysts

Changes in AC electrical conductance G of vanadium substituted 12-molybdophosphoric acid and of its (mono)-acid and neutral cesium salts were studied. Depending on the environment, the electrical conductivity is a combination of protonic and electronic conduction, the protonic one dominating at low temperature.     

Low temperature 13C NMR magnetic resonance in solids 4. Cyclopropane,bicyclo[1.1.0]butane and [1.1.1] propellane

The solid state ¹³C NMR spectra of bicyclo[1.1.0]butane and [1.1.1]propellane have been measured at low temperature. The orientation of the principal axes of the chemical shielding tensor have been determined with ab initio calculations based on the IGLO (Individual Gauge for Localized Orbitals) method when they are not determined by symmetry. Excellent agreement is obtained between the calculated and experimental principal values of the shielding tensor when basis sets containing polarization functions are used. In most cases the agreement is such that the calculated values are within the experimental error.Part 3 of this series: Ref. [7]     

Über einige nebenprodukte der thermolyse von bis(trimethylsilyl)-diimin

In the course of decomposition of bis(trimethylsilyl)diimine (BSD), which leads mainly to five products, the tris(trimethylsilyl)hydrazyl radical is formed among other intermediates. This radical reacts with hydrogen donors HR (e.g. HR = solvent) to tris(trimethylsilyl)hydrazine and radicals ·R, which on the other hand react further with BSD to by-products of BSD thermolysis. The types of these by-products and mechanisms of their formation are discussed. The thermolysis of BSD in toluene, for example, produces tris- and bis(trimethylsilyl)benzylhydrazine and bis(trimethylsilyl)benzalhydrazone.     

Nonbonded interactions. 1. Anisotropic hydrogen-hydrogen interactions

The hydrogen-hydrogen nonbonded potential which may be derived from the calculated interactions between hydrogen molecules has been obtained. The best three-parameter Buckingham function gave an RMS error of 0.18 kcal/mol in fitting Price and Stone's ab initio data for 130 pairs of hydrogen molecules, which may be compared with an RMS error of 0.74 kcal/mol using the parameters in the MM2 force field. Burton's basis set is also considered. A better fit to these data requires that the angular relationship between the bonds be included. The data for hydrogen as well as experimental data for chlorine show that these atoms appear “larger” normal to the bond axis than along the axis, and this is probably also the case for other atoms. When simple angular terms are added it is possible to fit the Price and Stone data set with an RMS error of less than 0.06 kcal/mol. The preferred function was: V = [a₀ + a₁(sin θ₁ + sin θ₂)⁴ + a₂r]e^?3r ? [c₀ + c₁(1 + sinθ₁sinθ₂)]/r⁵. Deficiencies in the current ab initio data are discussed.     

Uranyl sorption onto gibbsite studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was combined with batch experiments to study the sorption of uranium(VI) onto gibbsite (gamma-Al(OH)3). The experiments were performed under ambient conditions in 0.1 M NaClO4 solution in the pH range from 5.0 to 8.5 using a total uranium concentration of 1x10(-5) M, and a solid concentration of 0.5 g/40 ml. Two uranyl surface species with fluorescence lifetimes of 330+/-115 and 5600+/-1640 ns, respectively, were identified. The first species was dominating the more acid pH region whereas the second one became gradually more prominent towards higher pH values. The fluorescence spectra of both adsorbed uranyl(VI) surface species were described with six characteristic fluorescence emission bands situated at 479.5+/-1.1, 497.4+/-0.8, 518.7+/-1.0, 541.6+/-0.7, 563.9+/-1.2, and 585.8+/-2.1 nm. The surface species with the short-lived fluorescence lifetime of 330 ns is attributed to a bidentate mononuclear inner-sphere surface complex in which the uranyl(VI) is bound to two reactive OH- groups at the broken edge linked to one Al. The second surface species with the significant longer fluorescence lifetime of 5600 ns was attributed to small sorbed clusters of polynuclear uranyl(VI) surface species. The longer fluorescence lifetime of the long-lived uranyl surface species at pH 8.5 is explained with the growing average size of the adsorbed polynuclear uranyl surface species.